Combined Cycle power plants employ gas turbines with Heat Recovery Steam Generators (HRSGs) that use the thermal energy in the exhaust from gas turbines to generate steam for power generation or process use. The large stationary gas turbines used in such power plants may typically have average exhaust gas velocities in the range of 200 ft/sec. The velocity of the gas turbine exhaust is not uniform however and some recent gas turbines have local exhaust gas velocities in the range of 660 ft/sec. HRSGs may have flow areas in the range of 5 to 10 times the gas turbines exit flow area and thus average entering velocities that are 5 to 10 times lower than those exiting the gas turbine. A diverging duct is therefore required to connect the gas turbine to the HRSG. A typical arrangement of the gas turbine exhaust diffuser, connecting duct and HRSG is shown in FIG. 1. It is desirable to locate the HRSG close to the gas turbine in a compact duct arrangement to minimize the area required for the power plant and to minimize the size and cost of the connecting duct. This can result in a high velocity jet of gas impacting the region of the front rows of heat transfer tubes in the HRSG that are in line with the gas turbine exhaust diffuser. Such high velocities can cause flow-induced vibrations that will damage the heat transfer tubes. The high aerodynamic loading on the tube banks can also cause movement of the entire front tube bank resulting in damage to components in and around the tube bank. The non-uniform velocities entering the HRSG front tube rows also reduce the heat transfer effectiveness of these rows.
In some cases flow controls have been used in the diverging duct to redirect flow within the duct and improve flow distribution to the front rows of tubes in the HRSG. These flow controls would be subject to very high aerodynamic loadings in a compact duct due to close proximity to the gas turbine. In addition to the steady aerodynamic loading, the flow controls are subject to dynamic loading due to the high levels of turbulence in the duct and thermal stress due to going from ambient temperature to the high gas turbine exhaust temperature. These issues make it unlikely that flow controls located in the diverging duct 36 will survive long-term operation.
As will be described in greater detail hereinafter, a structural array disposed upstream of the front tubes of an HRSG will overcome such problems, particularly when the turbine and HRSG are closely coupled.
Currently there is a need for an effective and reliable means for diffusing an exhaust stream 14 from a turbine to recover heat.